The present invention, in some embodiments thereof, relates to material analysis and, more particularly, but not exclusively, to the analysis of chemically-active material that undergoes a curing process when hardening, such as, but not limited to, cementitious material, e.g., concrete.
A chemically-active material often needs to be analyzed so as to determine the structural properties, e.g., strength and other physical-mechanical properties of the final cured product, such as its potential for shrinkage. The final strength of a chemically-active capillary-porous material is determined by the properties of the initial raw materials, mixing and compacting conditions, and specific composition such as mineral binder-to-aggregate ratio, water-to-cement ratio, water-to-aggregate ratio and the like [Neville A. M., “Properties of concrete,” Longman Scientific & Technical, 1981].
The hardening process of a chemically-active material can be considered as a series of consecutive transitions between different states of the material.
Initially, the material is a compaction structure who's physical and mechanical properties are determined mainly by compressive actions of capillary pressure on “water-air” boundaries. This state is characterized by an intensive development of the chemical reactions, such as hydration and hydrolysis and formation of gel (the term gel has been introduced into the scientific practice in conjunction to cementitious materials by T. Powers in an article entitled “The non-evaporable water content of hardened Portland cement pastes,” published in ASTM Bulletin, 1949, No. 158, and was further used by A. Neville in an article entitled “Properties of concrete,” published by Longman Scientific & Technical, 1961).
In a second state following the initial state, the material develops a coagulation structure, which is a capillary-porous colloidal body having chemically-active water-silicate dispersions.
In a third state the material develops a colloidal-crystalline structure, which is a quasi-solid capillary porous body. In this state the gel begins to age and crystalline structures are formed.
In a fourth state, the crystalline structures condensate, and the material has a solid capillary-porous body whose conditions are determined by the laws governing the interaction of particles and particle aggregates in the solid phase.
At any given state, the chemically-active material has a poly-dispersed structure of a moist capillary-porous body. The liquid phase of the material is therefore an informative component indicative of the porosity of the material and therefore of its strength. Water (both in a liquid and gaseous form) is always in a state of thermodynamic equilibrium with the porous solid phase with which it interacts. Thus, the properties of water are changing in strict accordance with structure formation and consequently with the strength growth of the hardening material. To this end, see, for example, Shtakelberg D. I. and Sithcov M. M., “Self-organization in disperse systems,” Riga, “Zinatne” Press, 1990; and Shtakelberg D. I., “Thermodynamics of water-silicate disperse materials structure-formation,” Riga, Zinatne, 1984; and Neville M. “Properties of concrete,” Longman Scientific & Technical. NT., 1988.
The duration of the above hardening process is typically rather long. For example, in cementitious materials typical duration of hardening is of order of one month, at which time the cement passes through all the above states and becomes a solid structure of a given compressive strength.
Due to the long duration of the hardening process, prior to reaching the final strength, the chemically-active material undergoes many complicated physical and chemical processes, which can essentially affect its physical properties. It is recognized that any change, deviation and non-observance of the technological regulations during preparation of the chemically-active material, such as ready-mixed or pre-cast concrete, may irreversibly reduce the properties (e.g., strength) of the final product. Reasons for poor final product quality include unexpected replacement of material suppliers, improper operation of the equipment or failure thereof and the like.
Hardening and strengthening of chemically-active material is initiated immediately once the compaction for a particular application is completed. However, many additional processes, affecting the final quality can takes place. For example, in case of concrete, the transportation of the mix from the manufacturing plant to the building site typically occurs between the preparation of the mix and the compaction thereof. Although during transportation the concrete mix is in a continuous motion inside a rotating drum so as to prevent setting or hardening, it is known that the final properties of cementitious products made after a prolonged transportation of the mix are different from the properties of the same products when made of a freshly prepared mix.
Prolonged transportation of the mix naturally extends the period in which chemical reactions such as hydration and hydrolysis occur. Thus, upon arrival to the construction site different transport durations result in different initial states for the hardening and strengthening processes evolve.
Other factors which are known to alter the hardening process include, chemical additives of various functional purposes, temperature conditions during hardening, curing conditions of the freshly formed concrete, non-homogeneity of the mix, complexity and duration of the manufacturing process and the like.
Traditionally, the hardening process of concerted is monitored mechanically, via one or more techniques, such as slump test, VeBe consistency test, setting time test, compressive strength test and the like.
In the slump test, a cone (called slump cone) is placed on a base and filled with concrete. The cone is lifted up, leaving a heap of concrete that slumps. The slump cone is placed on the base to act as a reference and the difference in level between its top and the top of the concrete is measured. The slump test is typically employed for fresh concretes during the first 3 hours from preparation of the concrete mixture. Fast slump (also referred to in the literature as “high” slump) is observed when the amount of physically bound water in the concrete is relatively high, and slow slump (also referred to in the literature as “low” slump) is observed when the amount of physically bound water in the concrete is relatively low.
In the VeBe (also known as VB) consistency test the work needed to compact the concrete is measured. A slump cone of concrete is placed on a table and the cone is lifted as in the slump test. The table is vibrated at a standard rate. The time taken for the concrete to be compacted is measured. Vebe times range from 1 second for runny concrete to more than 12 seconds for stiff concrete. The VeBe test is typically employed during the period of 3-10 hours from preparation of the concrete mixture.
In the setting time test, the time of setting of concrete is estimated by means of penetration resistance measurements on mortar sieved from the concrete mixture. The penetrating device is shaped as a needle or wedge and is known as a Proctor needle. The Proctor needle is typically employed during the period of 10-24 hours from preparation of the concrete mixture.
The compressive strength test is typically performed post-curing. Concrete specimen is placed in a pressing machine and subjected to a gradually increasing compressive pressure so as to determine the failure load. The compressive strength is defined as ratio between the failure load and the cross-sectional area resisting the load. Also known is the pull-off test wherein an axial pull-off force is applied to a steel plate fixed to the concrete surface; and the pin penetration test wherein a smooth pin is driven into the concrete and the depth of penetration and/or pull out strength are measured.
Li et al. [“Determination of Concrete Setting Time Using Electrical Resistivity Measurement,” Journal of Materials in Civil Engineering, Vol. 19, No. 5, 423-427 (2007)] disclose a technique for determining concrete setting time using electrical resistivity measurement. Two critical points are identified on the resistivity curve and the setting times are estimated as linear functions of the abscissa values of the critical points.
Other techniques are disclosed in International Publication Nos. WO2001/065282, WO2005/047891 and WO2005/124339, and U.S. Pat. Nos. 6,396,265, 7,181,978, and 7,225,682, the contents of which are hereby incorporated by reference.